Hollow bodies of the above described nature are used for containing substances or for other purposes, in which the characteristics of commonly used synthetic materials, particularly polyolefins do not fulfill all requirements. As a typical example, the permeability of the polyolefins in respect to a contained substance and/or a surrounding medium may be mentioned. Thus, the permeability of these materials may be of significance when they are used in the production of fuel tanks.
Synthetic materials do exist, the permeability of which, e.g. for gasoline, is so low that it does not exceed the permissible values. However, these synthetic materials are expensive and/or not possible to process directly. In many cases, they also lack the mechanical characteristics of the polyolefins which are particularly suitable for fuel containers, specifically for tanks to be incorporated in motor vehicles, due to their solidity, toughness, elastic malleability, etc.
For the above described reasons, gasoline tanks in particular are manufactured so that a carrier layer, which consists mainly of a polyolefin, is provided with a barrier layer which is more or less impermeable to the contained substance, and the thickness of which is generally significantly less than that of the carrier layer. This barrier layer can be applied on the inside and/or the outside. Since, due to the different characteristics of the carrier layer material on the one hand and the barrier material on the other hand, it is generally impossible to bond these sufficiently well to each other, it has proven necessary to provide an additional layer of another material between the carrier layer and the barrier layer, the sole purpose of this additional layer being to provide a means of adhesion between carrier layer and barrier layer. Thus, this adhesive layer must consist of a material which forms a sufficiently stable bond with both the carrier material and the barrier layer material, whereby it is possible that the bond, e.g. between carrier layer and adhesive layer is achieved by means of heat-sealing, while the bond between the adhesive layer and the barrier layer is of an adhesive nature.
Under any circumstances consideration must be given to the strength of the bond between the adhesive layer and the barrier layer, since if the barrier layer is applied on the outside, the possibility does exist that the contained substance will diffuse through the carrier layer located on the inside and accumulate between the adhesive layer and the barrier layer, whereby the resulting gas pressure will affect the external barrier layer. Under the effect of this gas pressure, the barrier layer must be prevented from separating from the carrier layer or the adhesive layer. This can be achieved if the barrier layer is applied as an internal layer. However, for reasons that will be discussed later, this is extremely difficult if not impossible.
According to one procedure of the nature described at the outset, a multi-layer preliminary product is first extruded in the shape of a tube portion, which is then expanded to form a container according to the common method of subjecting it to internal pressure within a blow mold. The extrusion equipment required for the production of this preliminary product will necessarily have a very complicated structure, since it must be provided with at least three extruders--one for the material constituting the carrier layer, one for the material forming the adhesive layer, and one for the material that forms the barrier layer.
However, a more significant disadvantage is inherent in the fact that it is impossible or extremely difficult to re-use the waste material which cannot be avoided in the production process. This also applies to rejected portions which will be impossible to avoid in many, if not in most, manufacturing procedures. Such waste material will occur in the extrusion/blowing process, in any case, at the ends of the tube-shaped preliminary product. In the case of hollow bodies with a complicated shape, particularly for instance, fuel tanks to be built into vehicles, it is frequently impossible to avoid the occurrence of rejected parts, whatever the manufacturing procedure might be; the extent of waste portion per preliminary product may be up to 50% of the latter.
In the known extrusion/blowing procedure where the initial preliminary product is composed of three simultaneously extruded layers, the lack of re-usability of the waste material--which, in the case of gasoline tanks, will occur in greater than normal quantity--and the reject parts can be traced back to the fact that the parts consist of different materials, so that no uniform product will result from a reprocessing of this material. Admittedly, it is generally possible to mix and re-use the polyolefin constituting the carrier layer and the material forming the adhesive layer, since these materials are so similar that the existing differences would not be significant in the reprocessing. On the other hand, it must be assumed that the materials used for the barrier layer are so different in their chemical structure and physical properties from the polyolefins normally used for the carrier layer, that the two materials cannot be mixed with each other. In practice, this means that with the known procedure, the reject and waste portions cannot be re-used, or can be re-used only to a very limited extent, which will necessarily make the manufacturing significantly more expensive.
Another difficulty may be encountered in connection with the heat-sealed seams which are unavoidable in production of hollow bodies according to the extrusion/blowing procedure and which are formed on the preliminary product as a result of the squeezing off (flashing) process. When the hollow mold, which generally has two parts, is placed around the preliminary product, it is normally closed at one end, whereby simultaneously unnecessary material is squeezed off. Squeezing (flashing) processes of this type with simultaneous formation of a heat-seal seam may also occur in other areas of the preliminary product whenever necessary for adjusting it to the shape of the final product, i.e. the hollow body to be manufactured.
If the carrier layer, which generally consists of polyethylene or another polyolefin, is applied on the inside, there are generally no difficulties in producing a flawless heat-seal seam capable of resisting all occurring operational stresses, since the materials that can be used for the carrier layer possess the corresponding properties. Furthermore, the thickness of the carrier layer is ordinarily significantly greater than that of the other layers, so that the heat-seal seam of the carrier layer alone is sufficient to fulfill all practical requirements, regardless of whether or not the other layers are more or less involved in the formation of the heat-seal seam.
However, if the carrier layer is placed on the outside, the unavoidable consequence is that facing areas of the interior barrier layer are pressed against each other and must be heat-sealed to each other simultaneously while squeezing off the excess material. However, the barrier layer is generally so thin that it cannot form a sufficiently strong heat-seal seam, not even if the material has good heat-sealing properties. This applies for the adhesive layer as well. With this arrangement of the layers, the external carrier layer is insufficiently or not at all involved in the formation of the heat-seal seam, since the other two layers, i.e. the adhesive layer and the barrier layer, are located between the two facing carrier layer areas which are to be heat-sealed to each other, and since the barrier layer cannot form a good heat-seal bond with the material constituting the carrier layer, due to reasons cited at the outset.
In order to circumvent these difficulties, the suggestion has already been made to perform the co-extrusion of the generally tube-shaped preliminary product in such a manner that the adhesive layer and the barrier layer are extruded only over a predetermined portion of the length of the preliminary product, such that the end regions, where the flashing and heat-sealing processes are to be performed, would consist of the carrier layer only. This would make it possible to re-use that part of the excess material which consists of one material only. However, the equipment to accomplish a complete procedure is extremely complicated and consequently expensive as well as liable to malfunction. In addition, it is normally not possible to extrude the preliminary product with sufficient precision to assure that a specific section of the preliminary product, provided with adhesive layer and barrier layer, will be exactly positioned from one work cycle to the next within the hollow mold where it is to be blown into a hollow body. Furthermore, a procedure of this nature would not make it possible to re-use the waste flash portions from the sides of the preliminary product.
The purpose of the invention is to modify, particularly to simplify, a process of the nature described at the outset, so that the disadvantages of known processes will be avoided. Thus, the limitations in respect to re-use of excess and waste material should be eliminated or at least significantly decreased. Another objective is to avoid or to significantly reduce the difficulties involved in producing a heat-sealed seam that would be sufficient under all practical conditions, specifically in those areas where a connection between two facing areas of the same material or the resulting hollow body is to be accomplished simultaneously with the squeezing off of excess material. The equipment for carrying out the process should be simple, at least not more complicated and expensive than the equipment used for carrying out known processes. In respect to selection of the synthetic materials to be used for a specific function or layer, an optimum choice should be feasible.